1. Field of the Invention
This invention is related to a multi-stage pump device which is used for feeding working fluid under pressure.
2. Description of the Related Art
A multi-stage pump device is shown in FIG. 4 as a pump device related to this invention. The multistage pump device shown in FIG. 4 comprises pump devices A, B, C, and D arranged in line to form a four stage pump device. Each pump device A, B, C, and D is firmly secured by a support frame 15. A casing 10 of the pump device A and a casing 20 of the pump device B are provided with a flange 12 and a flange 13, respectively. The flange 12 and the flange 13 are sealed by sealing means and firmly connected by a fastening belt 14.
The working fluid fed by the pump device A flows through a passage provided in the flange 12 and flows into the pump device B through a passage provided in the flange 13 of the pump device B. The working fluid thus flows into the pump B, and further flows into the pump device C through passage similarly provided in the flange (not numbered) of pump device C. As explained above, by arranging a plurality of pump devices in line, a high degree of vacuum can be obtained if used in a vacuum pump device, for instance.
In the conventional multi-stage pump device, however, each pump device is firmly secured by the supporting frame 15. Therefore stress occurs in the connecting area between the flange 12 and the flange 13 due to the thermal expansion of the casing 10 and the casing 20, respectively, by the rise of the temperature in the pump while the pump device is working. The stress may deform the casings 10, 20, and such deformation partially loads the bearings provided in the casings 10, 20 and shortens a life span of the bearings.